1. Field of the Invention
The present invention relates to a method of manufacturing an optical semiconductor device, and in particular relates to a collective manufacturing method of the optical semiconductor device.
2. Background Art
In order to cope with an increasing demand for communication systems, an optical communication system based on the WDM (wavelength divisional multiplexing) system is being developed, because it is capable of expanding the channel capacity of the optical fiber by transmitting light signals having different wavelengths in one optical fiber without extending the manufacturing facility. It is a matter of course that this WDM optical communication system requires a light source, which emits a plurality of lights having multiple wavelengths. Thus, the problem in this WDM optical communication system is to provide a light source laser which can emit a plurality of lights having multiple wavelengths.
Japanese Unexamined Patent Application, First Publication No. Hei 10-117040 discloses a collective manufacturing method of a semiconductor device in which different wavelength DFB (distributed feedback) lasers and different wavelength EA (electro-absorption) modulator integrated DFB lasers are integrated on a semiconductor substrate. In the above Japanese patent application, a method is used for providing multiple oscillating frequencies on a single semiconductor substrate by first forming diffraction gratings having different cycles (pitches) A as shown in FIG. 46 by electron beam exposure and by an etching technique and by forming a multiple layered structure including active layers (light absorbing layers) having multiple band-gap wavelengths corresponding to the oscillation wavelengths by a selective MOVPE (metal-organic vapor phase epitaxy) growth method. In this method, since the oscillation wavelength can be made to coincide to some extent with the band-gap wavelength of the laser active layer, the threshold for the laser oscillation or the homogeneity of the oscillation efficiency can be maintained comparatively consistent.
However, when the active layers (and light absorbing layers) are produced by selective MOVPE growth, the band-gap wavelength of the light guide layer formed on the diffraction gratings and the thickness of the active layers change. When the band-gap wavelength of the light guide layer changes, the absolute value of the refractive index of the layers on the diffraction gratings changes, which results in changing the periodic change of the refractive index by the diffraction grating. When the thickness of the active layer changes, a light confinement factor in the active layer changes, which results in changing the light intensity of the diffraction grating region. The variation of the periodic change of the refractive index of the guide layer by the diffraction grating and the light intensity of the diffraction grating region are parameters related to a coupling coefficient xcexa, the coupling coefficient xcexa changes as the oscillation wavelength changes in the collective formation of the multiple wavelength laser in which variations occur in both of the periodic change and the light intensity change.
The technique disclosed in the above-described Japanese patent application is to broaden the mask width of a pair of stripe-like masks for the selective growth, in order to lengthen the oscillation wavelength. However, there are two factors affecting the coupling coefficient xcexa: (1) the absolute value of the refractive index of the light guide layer becomes large because the band-gap wavelength of the light guide layer on the diffraction grating increases to a longer wavelength; and (2) since the thickness of the active layer increases, the light confinement coefficient in the active region increases and as a result, the light intensity in the diffraction grating region decreases. The effect of the above item (1) has an action to increase the coupling coefficient xcexa and the effect of the above item (2) has an action to decrease the coupling coefficient xcexa, so that the relationship between the oscillation wavelength (or the selection growth mask width) and the coupling coefficient xcexa changes depending upon the ratio of the magnitudes of the above effects of (1) and (2).
The ratio of the magnitudes of the above effects (1) and (2) is dependent on the MOVPE apparatus for growing the crystal or the growth conditions. The coupling coefficient xcexa is a parameter related to the oscillation threshold value or the light emission efficiency of the DFB laser, the single longitudinal mode yield, or the long distance transmission characteristics. Thus, if the crystal is not grown homogeneously, the manufacturing yield of the elements is decreased.
There is a trade-off between the longitudinal single mode yield and the effect of the coupling coefficient xcexa on the long distance transmission characteristic, and the provision of the optimized homogeneous xcexa value is an important factor for obtaining a high final yield.
Therefore, the first problem is that the longitudinal single mode oscillation yield decreases as a result of the heterogeneity generated in the laser oscillation threshold currents and in the light emitting efficiencies at various wavelengths. This is caused due to the fact that the light emitting elements emitting lights with different wavelengths from each other have different coupling coefficients.
The second problem is that, when each laser light is transmitted for a long distance, the transmission characteristic yield for a laser light changes depending on the wavelength of the laser light. Since the coupling coefficient of each laser oscillating element changes, the wavelength variation (wavelength chirp) for each laser oscillating wavelength by residual reflection at each end surface changes for each laser element.
The object of the present invention is to suppress the dispersion of the coupling coefficients of the DFB laser portion, which is usually caused at the time of collective forming of the different wavelength DFB laser or the different wavelength EA modulator integrated DFB laser on a semiconductor substrate, and the object of the present invention further extends to homogenization of the laser oscillation threshold value, the light emitting efficiency, and the long distance transmission characteristics, and to the improvement of the longitudinal single mode oscillation yield.
The object of the present method for manufacturing the optical semiconductor device is to provide a method which is capable of solving the problems occurring in the conventional methods for collectively forming the different wavelength DFB laser. That is, the object of the present invention is to provide a method for manufacturing the different wavelength DFB laser or the different wavelength DFB laser integrated element, which is capable of maintaining the coupling coefficient xcexa at a constant value, even if the oscillating wavelength differs.
The present invention has been carried out to overcome the above-described problems and the following technical constitution has been obtained.
The present invention provides an optical semiconductor device comprising a plurality of semiconductor lasers formed on a single substrate, wherein-each of said semiconductor lasers emits laser lights having oscillating wavelengths differing from each other by different cycles of a plurality of diffraction gratings, wherein the composition of an optical guide layer in contact with one of said diffraction gratings is determined such that the coupling coefficient of each of said semiconductor lasers is maintained at a constant value.
The present invention also provides an optical semiconductor device comprising a plurality of semiconductor lasers formed on a single substrate, wherein each of said plurality of semiconductor lasers emits longitudinal single mode laser lights having different oscillating wavelengths due to a distributed feedback operation of a periodic change of the refractive index in the semiconductor lasers, and wherein each of said plurality of semiconductor lasers have the same coupling coefficient by being provided with a diffraction grating embedded semiconductor layer each having a refractive index corresponding to the oscillating wavelength.
In the above optical semiconductor device, each of said plurality of semiconductor lasers comprises a diffraction grating embedded semiconductor layer made of InGaAsP having a band-gap wavelength (energy) corresponding to the oscillating wavelength thereof, and each of said semiconductor lasers is a distributed feedback semiconductor laser.
In the above optical semiconductor device, an optical modulator is monolithically integrated with said semiconductor laser.
In the above optical semiconductor device comprising a plurality of semiconductor lasers comprising an InGaAsP guide layer formed on or below said diffraction gratings, a multi-quantum well layer, and an InP clad layer on an InP substrate, and which emit laser lights having different wavelengths determined by the cycle of said diffraction gratings, the refractive index of said guide layer is adjusted so as to equalize the coupling coefficients of the respective semiconductor lasers.
The present invention also provides a manufacturing method for collectively manufacturing, on a single substrate, an optical semiconductor device comprising a plurality of semiconductor lasers which emit longitudinal single mode laser lights having different wavelengths due to a distribution feedback operation of a periodic change of the refractive index in respective semiconductor lasers, wherein the refractive indexes of said diffraction grating embedded semiconductor layer are decreased (or increased) so as to cancel the difference of the coupling coefficients of the respective semiconductor lasers whose coupling coefficients are increased (or decreased) when the diffraction gratings for generating a distribution feedback operation are formed in the same configuration and the refractive indexes of said diffraction grating embedded semiconductor layers are fixed at the same value.
In the above manufacturing method for collectively manufacturing, on a single substrate, an optical semiconductor integrated device comprises a plurality of semiconductor lasers which emit longitudinal single mode laser lights having different wavelengths due to a distributed feedback operation of a periodic change of the refractive index in the respective semiconductor lasers, and a plurality of optical semiconductor portions integrally formed with said semiconductor lasers for receiving respective laser lights from said plurality of semiconductor lasers, wherein the refractive indexes of said diffraction grating embedded semiconductor layers are decreased (or increased) so as to cancel the difference of the coupling coefficients of the respective semiconductor lasers whose coupling coefficients are increased (or decreased) when the diffraction gratings for generating a distributed feedback operation are formed in the same configuration and the refractive indexes of the diffraction grating embedded semiconductor layers are fixed at the same value.
In the above manufacturing method, said optical semiconductor integrated device comprises longitudinal single mode oscillating semiconductor lasers and optical modulators.
In the above manufacturing method, the band-gap wavelengths of said diffraction grating embedded semiconductor layers are made shorter (or longer) so as to cancel the difference of the coupling coefficients of the respective semiconductor lasers whose coupling coefficients are increased (or decreased) when the diffraction gratings for generating a distribution feedback operation are formed in the same configuration and the band-gap wavelength of said diffraction grating embedded semiconductor layers are fixed at the same value.
In the above manufacturing method, said diffraction grating embedded semiconductor layer is made of InGaAsP, and the change of the refractive index of said InGaAsP layer is executed by changing the compositional ratio of In and Ga in Group III.
In the above manufacturing method, said diffraction grating embedded semiconductor layer is made of InGaAsP, and the change of the band-gap wavelength is executed by changing the compositional ratio of As and P in Group V.
In the above manufacturing method, the method for changing the refractive index or the band-gap wavelength of said diffraction grating embedded semiconductor layer is a selective metal organic vapor phase growth method.
In the above manufacturing method, the method for changing the refractive index or the band-gap wavelength of said diffraction grating embedded semiconductor layer is provided by adjusting a flowing ratio of group V group materials in an atmospheric pressure double-fluid layer type metal organic vapor phase epitaxy method.
An optical communication module is provided by assembly of the above described semiconductor device or the semiconductor device made by the above manufacturing method with a waveguide device for guiding an output light from said optical semiconductor device to the outside, a mechanism for inputting the output light from said semiconductor device to the waveguide device, and an electrical interface for driving said semiconductor device.
An optical communication apparatus is provided by assembly of the above optical semiconductor device or the above semiconductor device manufactured by the above-described manufacturing method with an optical transmission device loaded with said optical communication module and a receiving device for receiving the output light from said light transmission device.